Large networks of wireless communication nodes are expected to play an increasingly important role in networking sensors or actuators for a variety of applications. These applications include, but are not limited to, seismic monitoring, precision agriculture, environmental and atmospheric monitoring, automated electricity, gas and water meter reading, industrial control and automation, etc. In all such networks, a wireless communication node is attached to a sensing or actuating device. Each wireless communication node will have limited range and will only be able to communicate with a few other nodes directly. In all such networks, the ad-hoc formation and maintenance of the network as new nodes join the network and nodes leave the network due to failure or removal is clearly a requirement. The routing of messages from any source node in the network to any other destination node over multiple intermediate nodes or hops is also a further requirement.
Further, all such networks also provide an access point or a gateway device which acts as an interface between the network of wireless communication nodes and a Wide Area Network (WAN). These gateway devices are typically provisioned with a radio device that allows it to form a part of and participate in the wireless network of nodes attached to sensors as well a network interface device that allows it to also form a part of and participate in a WAN. Examples of WANs include the Public Switched Telephone Network (PSTN), the Internet, the cellular communication network etc. Accordingly, the gateway device provides connectivity to the wireless network of sensors and actuators from a remote location. In this way, the data messages generated by the nodes can be communicated through the gateways to a central server or computer where they can be stored in a database for further processing or used to generate alarms or signals to other devices and systems. The reverse is true as well; commands generated by the central computer or server, either automatically or based on human interaction, are communicated through the gateways to the wireless node or nodes that they are directed to, where appropriate action such as reading the sensor or performing some control measure can be taken. As such, the gateways form an integral and critical part of the wireless network of sensors/actuators.
Many methods can be found in the art which relate to the problem of automatically forming and maintaining the wireless sensor network and to the problem of routing messages within such a network. Examples include U.S. Pat. No. 5,987,011, issued to Toh, U.S. Pat. No. 6,078,269, issued to Markwell, et al., and U.S. Pat. No. 5,553,094, issued to Johnson, et al., all of which are incorporated herein for reference. A very flexible and powerful method for the automatic formation of and dynamic routing of messages within such a network is described in U.S. Pat. No. 7,035,207, issued to the same inventors and assignee of the present application, and incorporated herein for reference.
A second aspect of a wireless sensor network relates to the problem of scalability as it pertains to data traffic. In particular, since the gateways are conduits through which most, if not all, of the data in a sensor network is conducted to and from a central location, the gateways constitute a bottleneck in any such network. This problem can be tackled by provisioning a sufficient number of gateways to handle all the data traffic. Combined with a flexible means of forming and maintaining the network, e.g., as described in U.S. Pat. No. 7,035,207, a powerful networking system can be built whereby the nodes can choose from among multiple gateways to transmit their data to, therefore, mitigating the problem of data traffic bottleneck at the gateways. Moreover, such a system also provides an increased measure of reliability since the nodes can switch automatically to a different gateway if any particular gateway device fails.
However, although such a system largely solves the problem of scalability as it relates to data traffic, it still contains an inherent shortcoming, relating to the problem of scalability as it pertains to storage of routing tables at the nodes. In a very large network containing hundreds of thousands of nodes, if each node were to maintain a flat routing table allowing it to route to any other node, the size of such a routing table becomes prohibitively large. One possible solution to this problem would be to manually split the whole network into multiple segments or sub-networks centered around each individual gateway. The nodes in any one sub-network would store in their routing tables only entries for other nodes in the same sub-network, consequently reducing total size of the routing tables. In such a scheme, the nodes in a sub-network can only route to other nodes in the same subnet; if they need to reach a node in another sub-network, they are required to communicate to the gateway for that sub-network, which can then route to other subnets.
Many such manual means of dividing the network into sub-networks are known in the art. For example, the well-known Internet Protocol (EP) allows the use of subnet masks, therefore letting a network administrator to perform manual configuration in order to divide the network into subnets. However, the use of such a subnet in large-scale wireless sensor networks introduces a shortcoming, which is that the nodes can no longer switch to a different gateway if required. As mentioned earlier, this capability is very desirable in order to provide a high degree of reliability when a gateway device fails, or when wireless link conditions change. There is then, a need for a system and method by which a large wireless sensor/actuator network can automatically segment itself into smaller sub-networks but at the same time allow each node in the network to retain the ability to join a sub-network as well as to switch to a different sub-network as needed.